Ignition

P031B — Ignition C Control Signal Circuit Low

Severity: medium Drive with care, to a shop

What P031B means

The engine control unit sends a control signal to an ignition coil or its built-in driver, and that signal should stay within a defined voltage range. P031B points to a low voltage condition in ignition control circuit "C". The exact definition varies by manufacturer, and the code may be listed as reserved in the standard, so scan tool descriptions differ. Check the wiring diagram to see which coil or cylinder is assigned to circuit C.

Symptoms

  • Check Engine light on
  • Rough idle
  • Hesitation or jerking during acceleration
  • Misfires that a scan tool may show
  • Higher fuel consumption
  • Hard starting

Possible causes

  1. Ignition control signal wire shorted to ground
  2. Open circuit or poor connection in the harness or connector
  3. Faulty ignition coil or its built-in driver
  4. Corroded or damaged coil connector terminals
  5. Low coil supply voltage from a blown fuse or bad relay
  6. PCM fault, which is uncommon

How to diagnose

  1. Read all codes and freeze frame data, and look for misfire codes (P030x).
  2. Inspect the coil connectors and harness for chafing, oil, corrosion or melted insulation.
  3. Check the fuses and relays that feed the ignition system.
  4. Swap the circuit C coil with a neighboring one and see whether the fault follows.
  5. Use a multimeter to measure power, ground and the control signal at the coil connector, following the vehicle wiring diagram.
  6. Test the signal wire from the PCM to the coil for opens and shorts to ground.
  7. Check the control signal waveform with an oscilloscope.
  8. If the wiring and coil are good, test the PCM and its power supply.

Common fixes

  • Repair or replace damaged wiring and connectors
  • Replace the faulty ignition coil or driver module
  • Replace the fuse or relay if power supply is the problem
  • Reflash or replace the PCM, only after ruling out everything else

Questions and answers

What does P031B mean?

The PCM detected a low signal in ignition control circuit "C". The official code list may mark it as reserved, so confirm the definition for your specific make.

Can I drive with P031B?

Only for a short distance and under light load. If the engine runs rough or the Check Engine light flashes, stop, because misfires can damage the catalytic converter.

How do I fix P031B?

Start with the wiring and coil connector, then test the coil by moving it to another cylinder. Replacing the PCM should be the last step.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.